Mechanisms of recovery from leukemia induced by retroviral infection have been studied in the Friend murine leukemia virus system. Viral genes in both the helper and defective components of Friend virus complex have been examined to determine their respective roles in generation of and recovery from erythroleukemia . In addition, host genes influencing the anti-viral immune response including those in the major histocompatibility complex (H-2A, E, D, and T) and non-MHC genes such as Rfv-3 have also been studied to identify the mechanisms of elimination of both virus and leukemia cells. In a separate aspect of the project retrovirus-induced brain disease has been studied using polytropic retroviruses derived from Friend murine leukemia virus. This work has focused on analysis of the cell specificity of virus infection and the pathology produced in the central nervous system. Furthermore, the role of various regions of the viral envelope gene in mediating disease has been studied using detailed mapping studies.